The invention relates to chemically modified cellulosic materials that can have improved properties such as wet strength, softness, absorbency, absorbency rate and others. The invention relates to a chemically modified cellulosic product and to a process for improving the cellulosic material.
Wet laid cellulosic fibers that are untreated prior to sheet formation typically have substantially unacceptable properties for use in towels, wipes and tissues. Important properties include wet strength, softness, absorbency, absorbency rate, etc. In other words, the wet strength of the material can be such that, upon immersion in water, paper can lose a great deal of its tensile strength in both sheet dimensions, can become a pulpy unstable mass, can have no tissue softness as that term is understood, can have very little absorbency or can have a very low absorbency rate until saturation is reached. Such sheet-like materials have little or no attractiveness to consumers in the market because of a substantial lack of utility in many operations in which the wet strength and absorbency of the tissue paper or wipe is of critical importance.
The use of additives to improve the properties of wet laid sheets is well known. Such additives include sizing agents, dry strength additives, wet strength additives, surface treatments, coatings, and all are well known. Such materials include rosin based sizing materials, cellulose reactive sizing materials, wax emulsions, fluorochemicals and others. Dry strength additives are typically polymeric materials including such compositions as polyacrylamides, vegetable gums, starches and others. Wet strength additives are commonly understood to be urea-formaldehyde resins, melamine-formaldehyde resins, amino-polyamide epichlorohydrin resins, polymeric amine epichlorohydrin resins, aldehyde modified resins and others. Cellulosic web surface treatments typically include pigments, resin coatings and lamination sheets.
One type of wet strength enhanced material is a carboxymethyl cellulose polymer. Such materials are used as add on additives applied directly to a typical cellulosic sheet. Carboxymethyl cellulose is often used with a type one type polymer such as a polyalkylene polyamine or a polyamido amine that can be post reactive with epichlorohydrin to produce a useful additive material. The application of sodium carboxymethyl cellulose with other cationic additives to a cellulosic sheet is one useful wet strength additive strategy that has some measure of success. Such processes are disclosed in Miller et al., U.S. Pat. No. 5,525,664 and Espy, U.S. Pat. No. 5,316,623. Further, Griggs et al., U.S. Pat. No. 3,103,462, teach a partially acylated cellulosic fiber which is followed by the use of a cationic thermosetting resin. Lask et al., U.S. Pat. No. 4,248,595, teach a crosslinked fiber. The cellulose is converted using a carboxyalkylating etherifying agent and then reacted with a crosslinking agent in an aqueous medium. Chene et al., U.S. Pat. No. 3,657,066, teach a wet strength carboxylated cellulosic material. In Chene et al., the underlying cellulosic fiber is oxidized to produce carboxyl groups which are then crosslinked with a melamine formaldehyde resin. These references are primarily directed to crosslinked materials that have covalently bonded crosslinking agents that directly bond one cellulosic fiber moiety to a second cellulosic fiber moiety through substantially increase. the molecular weight of the resulting material. This also is an accepted regimen for improving the properties of the cellulosic materials. While this is a useful process, the cost and properties of the resulting product can be a problem in the marketplace.
For the purpose of this patent application, the term xe2x80x9ccarboxymethyl cellulose materialxe2x80x9d indicates a cellulosic fiber that has been modified with a chemical reagent to introduce carboxymethyl cellulose ether groups bonded directly onto a hydroxyl site which introduces a terminal carboxyl group into the cellulosic moiety. For the purpose of this patent application, a cationic additive material, whether small molecule-monomeric or polymeric, is a positively charged nitrogen containing additive material that ionically associates with carboxymethyl cellulose groups in the paper product. The materials disclosed in this application and the products of the processes of the application are not covalently crosslinked into molecular weight cellulosic materials. The association of the carboxymethyl groups ionically with the cationic additive materials enhances the physical properties of the materials without covalent bonding.